a planning method for evaluating downstream effects of detention basins

VOL. 17, NO. 5 WATER RESOURCES BULLETIN

AMERICAN WATER RESOURCES ASSOCIATION OCTOBER 1981

A PLANNING METHOD FOR EVALUATING DOWNSTREAM EFFECTS OF DETENTION BASINS’

Mark E. Hawley, Timothy R. Bondelid, and Richard H. MccUen’

ABSTRACT: While storm water detention basins are widely used for controlling increases in peak discharges that result from urbanization, recent research has indicated that under certain circumstances detention storage can actually cause increases in peak discharge rates. Because of the potential for detrimental downstream effects, storm water manage ment policies often require downstream effects to be evaluated. Such evaluation requires the design engineer to collect additional topographic and land use data and make costly hydrologic analyses. Thus, a method, which is easy to apply and which would indicate whether or not a detailed hydrologic analysis of downstream impacts is necessary, should decrease the average cost of storm water management designs. A planning method that does not require either a large data base or a computer is presented. The time coordinates of runoff hydrographs are estimated using the time-of-concentration and the SCS runoff curve number; the discharge coordinates are estimated using a simple peak discharge equation. While the planning method does not require a detailed design of the detention basin, it does provide a reasonably accurate procedure for evaluating whether or not the installation of a detention basin will cause adverse downstream flooding. (KEY TERMS: detention; flood control; hydrology; planning; reser- voirs; storm water management.)

INTRODUCTION

Storm water management (SWM) basins are the most widely used means of controlling the rate of runoff from developed areas. In many jurisdictions, SWM policies have been established with the intent of limiting the detrimental effects of urbanization on downstream areas. Usually, the policy requires that the peak runoff rate from the urbanized area be controlled so that the peak discharge rate after development does not ex- ceed the predevelopment peak discharge rate. Unfortunately, limiting the peak discharge rate in this manner does not neces- sarily ensure that the detrimental effects at downstream points are minimized (McCuen, 1974,1979).

Urbanization decreases the natural storage capacity of a watershed; therefore, the volume of direct runoff from a given depth of rainfall increases with development. Urbanization also has the effect of decreasing the time required for water to flow from remote points to the watershed outlet; that is, the time of concentration is inversely proportional to the degree of urbanization. Thus, urbanization affects both the volume and

the timing of the runoff hydrograph. While SWM policies are designed to limit the effect of increases in peak discharge rate, they tend to ignore the effects of urbanization on the time characteristics of both direct runoff and flow through the SWM basin.

McCuen (1979) has shown that in some circumstances the change in timing caused by a SWM basin can result in increased downstream flooding. This type of situation is illustrated in Figure 1. In the predevelopment condition, the peak discharge rate from area 2 passes point A before the peak from area 1 arrives. After development, the peaks arrive at point A at nearly the same time; this causes constructive interference and results in a larger total peak discharge rate at point A, even though the peak flow from area 2 has not been increased over that which occurred prior to urbanization. Because of this possibility, the timing of the SWM basin outflow hydrograph should be considered when a SWM basin is to be used for controlling runoff from an area that is to be developed.

If SWM policies required all SWM basin analyses to include measuring the downstream effects, the cost of SWM design would increase substantially. In addition. to the computer costs, it would be necessary to compile hydrologic, land use, and soil data bases for the downstream area. Because SWM will not always have a detrimental downstream effect, the average project costs would be decreased if a simplified method that would indicate whether or not a SWM basin would have a detrimental downstream effect were available; that is, a method is needed that can be used in the planning stage of site develop ment t o indicate whether or not downstream effects should be incorporated into the design of a SWM basin. The intent of this paper is to present such a method and illustrate its applica- tion.

DEVELOPMENT OF THE PLANNING METHOD

Analysis of Planning Requirements The best way to determine the downstream effects of a SWM

basin is to compare the predevelopment and postdevelopment

‘Paper No. 81036 of the Water Resources Bulletin. Discussions are open until June 1, 1982. ‘Respectively, Faculty Research kssistant and Graduate Research Assistant, Department of Civil Engineering; and Associate Dean, College of Engineer-

ing; University of Maryland, College Park, Maryland 20742.

806 WATER RESOURCES BULLETIN

Hawley, Bondelid, and McCuen

hydrographs at downstream points. In the planning stage it is not necessary to have exact hydrographs; reasonably accurate estimates of the hydrographs should be sufficient. The hydrograph at a downstream point, such as point A in Figure 1, is the sum of the hydrographs from the upstream areas 1 and 2. The predevelopment hydrograph at A is the sum of the predevelopment direct runoff hydrographs from areas 1 and 2, while the postdevelopment hydrograph at A is the sum of the direct runoff hydrograph from area 1 and the SWM basin outflow hydrograph from area 2. Thus, the planning method consists of nothing more than a means of quickly estimating three hydrographs, the predevelopment direct runoff hydrograph from area 2, the direct runoff hydrograph from area 1, and the SWM basin outflow hydrograph representing the postdevelopment hydrograph from area 2.

C l t D t V l L O C M t N T H V O I O O I A C H #

t

Figure 1. Illustration of the Possibility of Increased Peak Discharges Due to SWM Basins.

In order to simulate these hydrographs quickly, estimates of both the timing coordinates and the discharge coordinates are required. Two models that provide these estimates are the SCS TR-20 model and the TR-55 tabular model. The TR-20 model requires a considerable data base and a computer, and is therefore not practical for preliminary planning purposes. The TR-55 tabular method is less complex than TR-20 and can be performed by hand, but it is not capable of estimating SWh4 basin outflow hydrographs. The SCS methods were designed to simulate natural storage conditions rather than man-made

807

SWM basins. Examination of a few SWM basin hydrographs showed that the characteristic shape of a SWM basin outflow hydrograph is considerably different from the characteristic shape of a direct runoff hydrograph. Examples of these two hydrograph shapes are shown in Figure 1. A SWM basin outflow hydrograph generally has a smaller slope on the rising limb and an extended period during which flows are at or near the peak rate; the shape of the basin outflow hydrograph is more nearly trapezoidal than the triangular shape of a direct runoff hydrograph. For this reason, separate models are required for estimating the SWM basin outflow and direct runoff hydrographs.

Storm water management policies often require SWM basins to control the postdevelopment runoff so that the peak discharge is no greater than the predevelopment peak discharge. Therefore, the predevelopment and postdevelopment peaks can be considered equal; the TR-55 graph method is a fast and widely used method of estimating peak discharge, although it does not provide any estimates of the timing characteristics of the runoff. To be consistent, it is best to use the same techniques for estimating both direct runoff and SWM basin outflow hydrographs; therefore, the logical method is to use the TR-55 graph method for estimating the peak discharge rates and to develop empirical equations for estimating the time coordinates of all the hydrographs. Because of the fundamental differences between the two types of hydrographs, it was not possible to develop one set of timing equations that would work for both types; therefore, separate sets of timing equations were developed for the runoff and SWM basin outflow hydrographs.

Data Generation In order to develop equations for predicting time coor-

dinates of hydrographs, a data base encompassing a wide range of watershed conditions was required. Collection of a set of measured hydrologic data that included the proper mix of watershed conditions was not possible; therefore, a synthetic data base was generated. Because of the fundamental differences between the characteristics of the direct runoffhydro- graphs and the SWM basin outflow hydrographs, different methods were used for synthesizing these two types of hydrographs.

Synthesizing the Direct Runoff Hydrojpaphs. The data base for the direct runoff hydrographs was generated using the SCS TR-20 runoff model with a variety of watershed conditions. The usual data requirements for TR-20 include the area of the watershed, the average slope, the curve number, the hydraulic length, the time of concentration, and the depth of rainfall. The type I1 storm distribution with half-hour rainfall increments was used for all analyses in this study. The average slope and the hydraulic length are used only in computing the time of concentration; therefore, if the time of concentration is specified, then these data need not be included. The area does not affect the timing of the runoff hydrograph if the time of concentration is specified. Because the TR-55 graph method results in peak discharge in cfs per unit area rather than in cfs, it was not necessary to vary the watershed area in generating

WATER RESOURCES BULLETIN

A Planning Method for Evaluating Downstream Effects of Detention Basins

the data base, The remaining watershed variables are the curve number, the rainfall depth, and the time of concentration. Values of the curve number were 60,70, 80 ,90 , and 95;values of the rainfall depth were 2.8, 4.3, and 7.0 inches, which cor- respond to the 2-year, 10-year, and 100-year storms in the state of Maryland; values of the time of concentration ranged from 0.25 to 2.0 hours in quarter-hour increments. Thus, 120 combinations of values were used to generate 120 direct runoff hydrographs; these hydrographs form the data base used in developing the timing equations for the direct runoff hydrographs.

Generating the SWM Basin Outflow Hydrographs. The SWM basin outflow hydrographs were generated using the Bondelid method (Bondelid and McCuen, 1980). Tests showed that the timing of the SWM basin outflow hydrograph is determined almost entirely by the depth of rainfall and the watershed conditions. The peak outflow rate at any one site may be limited to the predevelopment peak by using many different SWM basin configurations, but all of these configurations will result in nearly identical outflow hydrographs. This is rational because the peak flow rate and the volume of basin storage required are not functions of the SWM basin design parameters (i.e., size and number of riser pipes, depth, side slope); rather, the SWM basin parameters are dictated by the peak flow rate and the volume of basin storage that is required. The peak flow rate is determined by the predevelopment watershed conditions, and the volume of basin storage is determined by the differences between the predevelopment and postdevelopment watershed conditions. Therefore, the SWM basin design parameters have very little effect on the basin outflow hydrograph as long as the set of basin design parameters constitutes a feasible solution. Table 1 illustrates this point by showing the timing of the outflow hydrograph for each of several different feasible SWM basin designs. The outflow hydrographs of the various basin designs exhibit almost no variation in timing.

Because the outflow hydrograph is not sensitive to the basin parameters, the Bondelid program for generating feasible SWM basin designs was modified to give only one feasible solution for each combination of watershed conditions. The five watershed conditions that were varied in creating this data set are the watershed area, average slope, rainfall depth, predevelopment curve number, and postdevelopment curve number. The predevelopment and postdevelopment times of concentration were calculated using the SCS lag formula:

in which tc is the time of concentration (hrs), L is the computed hydraulic length (ft), Y is the average slope (percent), and S is related to the curve number by the function:

S = 1000 -10 (curve number)

The computed hydraulic length, L, is derived from the watershed area using the function:

L = 2 0 9 ~ ( a r e a ) ' . ~ (3)

All of these equations are designed for use with SCS hydrograph models (SCS, 1972) and are compatible with the Bonde- lid method of designing SWM basins. The values of the watershed area that were used are 20, 60, and 100 acres; values of the predevelopment curve number were 60, 70, and 80; postdevelopment curve numbers ranged from 65 to 90 in increments of 5 and were always greater than the predevelopment curve number; values of the average slope were 1, 3, 5, and 7 percent; the rainfall depths were 2.8,4.3, and 7.0 inches. Sub- stituting the various combinations of these values into the

TABLE 1. Timing of SWM Basin Outflow Hydrographs From Various Basin Designs.

Time (hrs) From Start of Precipitation Riser Diameter Volume of Storage Surface Area Depth*

W P (ft) (acre-feet) (acres) (ft) T50R T75R TP T75F T50F

1 3.50 2 3.25 3 3.00 4 2.75 5 2.50 6 2.25

3.048 9.599 0.758 12.0 12.2 12.8 14.4 15.7 3.167 9.077 0.802 12.0 12.2 12.8 14.5 15.8 3.187 8.305 0.867 12.0 12.2 12.8 14.5 15.8 3.199 7.437 0.952 12.0 12.2 12.8 14.5 15.8 3.149 6.353 1.085 12.0 12.2 12.8 14.5 15.7 3.080 4.971 1.294 12.0 12.2 12.8 14.6 15.7

Watershed Conditions:

Watershed Area = 100 acres. Redevelopment Curve Number = 70. Postdevelopment Curve Number = 80. Redevelopment Time of Concentration = 1.0 hours. Postdevelopment Time of Concentration = 0.5 hours. Rainfall Depth = 2.8 inches.

*Depth is maximum height of water surface above top of riser.

808 WATER RESOURCES BULLETIN


Bondelid program resulted in a data set of 324 SWM basin outflow hydrographs.

Developing the Timing Equations An examination of the direct runoff hydrographs showed

that they could be approximated with the required degree of accuracy by a triangular hydrograph; three points on the hydrograph had to be located, then joined by straight lines. The SWM basin outflow hydrographs are much flatter than the direct runoff hydrographs; therefore, five points were used in estimating the basin outflow hydrographs. Typical estimated hydrographs of both types are shown in Figure 2.

" O I O * O A

Figure 2. Estimated Hydrographs for Example of Planning Method.

Peak discharge rates for the estimated hydrographs were estimated using the TR-55 method; equations that can be used to estimate the timing coordinates of the hydrographs were developed. These equations were developed by using regression techniques with the generated hydrograph data base and are referred to as the timing equations.

Timing Equations for Direct Runoff Hydrographs. The first step in deriving the timing equations for the direct runoff hydrographs was to separate the synthesized data into three groups on the basis of the depth of rainfall. The hydrographs were synthesized for design storms of 2.8, 4.3, and 7.0 inches because these are the depths of the 2-year, 10-year, and 100- year design storms in Maryland. Separate equations were developed for each rainfall depth in order to determine whether one set of equations could be used for all return periods. A stepwise regression was then performed with each data set. The criterion variables were the time to peak (T,), the time to 50 percent of peak on the rising limb (T~oR) , and the time to 50 percent of peak on the falling limb (T~oF). The predictor variables were the curve number (CN), the time of concentration (tc), the depth of runoff (RO), and the peak flow rate (Qp). Correlation matrices of the predictor and criterion

809

variables were calculated separately for each of the 2-year, 10-year, and 100-year design storms.

Analysis of the results showed that the correlations between predictor and criterion variables were very similar for all three design storms. In each regression, the first variable to enter the equation was the time of concentration; the curve number was always the second variable to enter. Because the differences in the rainfall depths of the three design storms did not make a significant difference in either the correlation between the variables or the regression coefficients, the data set was recom- bined and all 120 observations were used with the stepwise regression program. The resulting timing equations for the direct runoff hydrographs are shown in Table 2, along with goodness- of-fit statistics; these equations are applicable to 24-hour type I1 design storms of any return period. The very high correlation coefficients and low standard errors of estimate indicate that these three points (Tp, T ~ o R , and T ~ o F ) can be predicted very accurately within the range of the calibration data.

Timing Equations for the SWM Basin Outflow Hydro- graphs. Equations were developed for estimating the times to five discharge stages of the basin outflow hydrographs. These five equations estimate the times to reach 50 and 75 percent of peak discharge on the rising limb ( T ~ o R and T 7 5 ~ , respectively), and time to peak (Tp), and the times to reach 75 and 50 percent of the peak discharge on the falling limb ( T 7 5 ~ and T ~ o F , respectively). The equations were developed by using stepwise regression techniques with the data set generated by the Bondelid method. The predictor variables that were available were: 1) the area, the average slope, and the computed hydraulic length of the watershed; 2) the predevelopment curve number, time of concentration, runoff depth, and peak discharge rate; 3) the postdevelopment curve number, time of concentration, runoff depth, and peak discharge rate that would occur if no SWM basin were installed; and 4) the change in the curve number, the rainfall depth, the volume of basin storage required, and the ratios of the predeveloprnent to postdevelopment runoff depths and the predevelopment to postdevelopment discharge rates. All 324 SWM basin outflow hydrographs were used as the data base; no distinction was made on the basis of rainfall depths because one set of timing equations was desired that would be applicable to any type I1 design storm depth. The timing equations that resulted from this analysis are shown in Table 3, along with goodness-of-fit statistics and the standardized partial regression coefficients.

The standardized partial regression coefficients in Table 3 are indicative of the relative importance of the predictor variables. On the rising limb of the hydrograph, the postdevelopment time of concentration is very important, while the runoff depth is considerably less important; both of these variables reflect the watershed conditions rather than the SWM basin design, so it seems that the timing of the outflow hydrograph on the rising side is almost entirely controlled by watershed characteristics. At the peak discharge and on the falling side of the hydrograph, the ratio of the postdevelopment peak discharge rate that would occur in the absence of a SWM basin to the predevelopment peak discharge rate is more important as a predictor variable than the depth of runoff. This ratio of


A Planning Method for Evaluating Downstream Effects of Detention Baslns

TABLE 2. Timing Equations and Goodness-of-Fit Statistics for the Direct Runoff Hydrographs.

Criterion Equation Correlation Standard Error of Standard Deviation of Coefficient Estimate (hours) Criterion Variable (hours)

T50R = 12.20278 + 0.30759 * tc - 0.00651 * CN 0.9559 0.06 (.8641) (-0.4089)

= 12.32515 + 0.65588 * tc - 0.00622 * CN 0.9702 0.10 TP (.9491) (-0.2012)

TSoF = 13.16962 + 1.36070 * tc - 0.01613 * CN 0.9452 0.28 (.9136) (-0.2421)

0.20

0.40

0.86

NOTES: tc := time of concentration (hours). CN = curvenumber. TSoR .= time to reach 50 percent of peak flow on rising limb (hours).

Tp TSoF = time to reach 50 percent of peak flow on falling limb (hours). Numbers inside parentheses are standardized partial regression coefficients.

= time to reach peak flow (hours).

TABLE 3. Timing Equations for SWM Basin Outflow Hydrograph Estimation.

Criterion Equation correlation Standard Error of Standard Deviation Coefficient Estimate (hours) of Criterion (hours)

~ ~~

= 11.79903 + 0.53128 * tcpost - 0.04274 * sost 0.9640 0.06 0.23 (0.8548) (-0.3063) T50R

= 11.90993 + 0.70941 * tcpost - 0.04727 * sost 0.9533 0.09 0.31 (0.8679) (-0.2575) T75R

= 11.39838 + 1.51487 * tcpost + 0.18329 * 9post/qpre 0.9387 0.29 0.83 TP (0.6848) (0.7337)

T75F = 9.95355 + 3.12555 * tcpost + 0.91341 * ‘Ipost/‘lpre 0.9591 0.92 3.24 (0.3613) (0.9349)

= 9.1829 +4.18501 * tcpost + 1.35179 * qpost/qpIe 0.9632 1.28 4.74 (0.3306) (0.9453)

T50F

NOTES: = postdevelopment time of concentration (hours). = postdevelopment depth of runoff (inches). = postdevelopment peak discharge rate without S W M basin (cfs). = predevelopment peak discharge rate (cfs).

tcpost

Qpost qpost qpre Numbers inside parentheses are standardized partial regression coefficients.

peak flow rates is indicative of the degree to which the basin must control the runoff hydrograph. Thus, the importance of this ratio can be considered as an indicator of the importance of the SWM basin characteristics in determining the timing of the SWM basin outflow hydrograph. The standardized partial regression coefficients show that as the rate of discharge decreases the SWM basin design becomes more important and the postdevelopment time of concentration becomes less important to the timing of the hydrograph.

The equations in Table 3 are suitable for use for any 24- hour, type I1 design storm depth because the effect of variation

81 0

in rainfall depth is included in each equation. In the first two equations the effect of rainfall depth is reflected in the postdevelopment depth of runoff. In the last three equations, the effect of rainfall depth is inherent in the ratio of the peak discharge rates, because the depth of rainfall is very important in determining the ratio. The change in the ratio with rainfall depth can be explained by reference to Figure 10-1 of Sec- tion 4 of the SCS National Engineering Handbook(SCS, 1972). For any pair of predevelopment and postdevelopment curve numbers, the ratio of the postdevelopment to the predevelopment runoff depth is much greater for small rainfall depths



than for larger depths. This effect is due to the greater relative importance of the initial abstraction in smaller storms. In the data base used in this study, the average value of the ratio of peak discharge rates was about 5 for a 2.8 inch design storm and about 2 for a 7.0 inch design storm. This indicates that smaller storms require more control if the SWM basin outflow peak discharge rate is to be no greater than the predevelop ment direct runoff peak rate.

PLANNING PROCEDURE The planning procedure consists of estimation and compari-

son of the predevelopment and postdevelopment hydrographs at a critical downstream point. The critical point is usually the point at which the runoff from the area to be urbanized (area 2 in Figure 1) joins the runoff from the undeveloped area (area 1 in Figure 1). This junction point (point A in Figure 1) is critical because channel flow from this junction tends to smooth out the hydrograph and decrease the peak discharge rate; therefore, if constructive interference between the area 1 direct runoff hydrograph and the area 2 SWM basin outflow hydrograph causes the postdevelopment peak discharge rate to be greater than the predevelopment peak, the effect will be most apparent at this junction point.

In estimating the predevelopment and postdevelopment hydrographs at the junction point the discharge rates are estimated using TR-55, while the timing coordinates are estimated using the equations in Tables 2 and 3. An important part of the planning process is the selection of a design storm; in many jurisdictions, legislation or established policy dictates this choice. Selection of a design storm is specific to each situation and will not be discussed in this paper. Once the design storm has been selected, which establishes the depth of rainfall, the planning procedure proceeds as described in the following paragraphs.

Estimation of Direct Runoff Hydrographs In using the TR-55 graph method for estimating peak dis-

charge rates, the required data are the watershed area, the rainfall depth, the curve number, and the time of concentration. A number of methods for estimating time of concentration are described in the TR-55 manual; all of these methods are compatible with the timing equations. Once the data have been collected, TR-55 can be used to estimate the peak flow rates. Then the equations in Table 2 can be used to estimate the time coordinates. The three points on the hydrographmay be located by the coordinate pairs ( T ~ o R , Qp/2), (T ,Qp), and T ~ o F , Qp/2). Connecting these points with straightyines, as in Figure 2, completes the estimation of the direct runoff hydrographs for the undeveloped area (area 1 in Figure 1) and for the predevelopment conditions on the area being developed (area 2 in Figure 1).

Estimation of SWMBasin Outflow Hydrograph The estimation of the SWM basin outflow hydrograph re-

quires knowledge of the postdevelopment curve number and time of concentration as well as the watershed area and rainfall

81 1

depth. These values are used with TR-55 to estimate the postdevelopment peak discharge rate from area 2 that would occur in the absence of SWM basin (qpost). Once qpost has been determined, the ratio qpost/qpre can be calculated. This ratio, the postdevelopment depth of runoff (Qpost), and the postdevelopment time of concentration (tcpOst) are substituted into the equations in Table 3 to estimate the time of coordinates of the SWM basin outflow hydrograph. Because the SWM basin peak discharge rate is to be controlled so that it is no greater than the predevelopment peak discharge from area 2 (qpre), qpre is used for the flow rate coordinates. The five points on the SWM basin outflow hydrograph have coordinates ( T ~ o R , qpre/2), (T75Rj 3qpre/4), (Tp, (Ipre), (t75Fp 3q /4), and ( T ~ o F , qpre/2); the lines connecting these points represent the estimated SWM basin outflow hydrograph.

Estimation of the Downstream Hydrographs The important hydrographs are not those for the individual

areas, but rather the downstream hydrographs formed by the addition of the individual area hydrographs. Figure 2 shows how the downstream hydrographs are generated. The predevelopment downstream hydrograph is simply the sum of the predevelopment area 2 direct runoff hydrograph and the area 1 direct runoff hydrograph. The postdevelopment downstream hydrograph is the sum of the SWM basin outflow hydrograph and the area 1 direct runoff hydrograph. In Figure 2 the installation of a SWM basin appears to increase the peak flow at downstream points significantly.

P'e

Example of the Planning Method To illustrate its use, the planning method was applied to

the Crabbs Branch watershed in Montgomery County, Mary- land. Figure 1 contains a schematic diagram of the watershed, the data and calculations appear in Table 4, and the estimated hydrographs are shown in Figure 2. The hydrographs in Fig- ure 2 indicate that the proposed development of area 2 and the installation of a SWM basin will result in an increased peak discharge rate at point A. The predevelopment peak rate is about 230 cfs and the postdevelopment peak is about 265 cfs, in- dicating an increase of roughly 15 percent. In order to sub- stantiate these results, the example was recomputed using TR-20 to generate the direct runoff hydrographs and the Bondelid method (Bondelid and McCuen, 1980) to generate the SWM basin outflow hydrograph. The results of thissimula- tion indicated an increase in peak discharge only slightly lower than that indicated by the planning method.

CONCLUSIONS On some watersheds the use of SWM basins to control run-

off from developing areas can increase discharge rates at downstream points rather than limiting discharges to those which occurred prior to development. In this paper, a quick planning method for estimating the potential for adverse downstream effects of a SWM basin has been developed. The planning method involves estimation of direct runoff hydrographs and SWM basin outflow hydrographs. The TR-55 graph method


TAB

LE 4

. W

orks

heet

for

Planning M

etho

d.

A2=

0

.07

CN

2=

58

tc

2= 0

. a

5

s2 = 7. aL

// Q

~=

0

.84

6

qu2

= 735

qp2'

4/. 6

Eq.

4 //.

oo

-

-

Eq.

5 4./3

-

m.6

12.57

Des

ignS

tonn

: R

etur

nper

iod

= /a

year

s

A3=

0

.07

CN

3=

81

tc3

=

0.1

4

= a.

396

Q~

= a.3

9

3 qu3=

/d

o0

qp3

= 16

7 a=

4-0

3

Eq.

7 /1

.75

Eq.

8 11

- p7

E

q.9

'2.4

9

Eq.

10 /3

.9J

~q

.

11 /

5T

dL

/

Var

iabl

e

A

= w

ater

shed

are

a (s

quar

e m

iles)

CN

= cu

rven

umbe

r

t, =

time

of c

once

ntra

tion (

hour

s)

S =

re

tent

ion

= (l

OO

O/C

N)-l

O

s 2

Iu

Q

=

runo

ff v

olum

e =

(P-0

.2s)

/(P

+o.S

S)

(inch

es)

qu

qp

a

=

qp3/

qp2

TsoR

=

T75

R =

Tp

=

tim

eto-

rise

toq

(hou

rs)

T75

F =

TsoF

= =

unit peak d

isch

arge

from

Fig

. 5-2

, TR

-55 manual (C

SW/in

ch)

= peak discharge =

A *

qu *

Q (c

fs)

time-

to-ri

se t

o 50

per

cent

of

q (h

ours

)

time-

to-r

ise

to 7

5 pe

rcen

t of

q (h

ours

)

P P

P

time-

to-f

all t

o 75

per

cent

of q

(h

ours

)

timet

o-fa

ll to

50

perc

ent o

f q

(hou

rs)

P P

5 ;;1 !

rn

C

rn

0,

Eq.

No.

T

imin

g Equations fo

r Area 1

1. T

50R

= 12

.202

78 +

0.3

0759

tcl -

0.00

651

CN

1

2. T

=

12

.325

15 +

0.6

5588

tcl - 0

.006

22 C

NI

P

3. T

50F'

13

.169

62 +

1.3

6070

tcl - 0

.016

13 C

N1

Eq.

No.

T

imin

g E

qs. f

or A

rea

2: R

edev

elop

men

t

4. T

soR

= 12

.202

78 +

0.3

0759

t,. -

0.00

651

CN

2

5. T

=

12

.325

15 +

0.65

588

tc2 - 0

.006

22 C

N2

P

6.

T5o

F=

13.1

6962

+ 1

.360

70 tc

2 - 0

.016

13 C

N2

Eq. N

o.

Ti

ig

Eqs.

for

Are

a 3:

Pos

tdev

elop

men

t

7. T

SoR

=

8. T

75R

'

P

11.7

9903

+ 0.

5312

8 tc

3 -

0.04

274

Q3

11.9

0993

+ 0

.709

41 tc

3 - 0

.047

27 Q

3

9. T

=

11.3

9838

+ 1

.514

87 tc

3 +

0.18

329 U

10. T

~~

~=

11.

TSo

F=

9.95

355

+ 3.

1255

5 tc

3 +

0.91

341 a

9.18

290

+4.1

8501

tc3

+ 1

.351

79 E

iii *Area 1

is th

e area u

pstre

am fr

om th

e ar

ea being d

evel

oped

. c

I-

r

m =! z

**A

rea

2 is

the

area

being d

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oped

.


is used for estimating discharge rates, and empirical timing equations are used for estimating the time coordinates of the hydrographs. The method was calibrated using simulated hydrographs developed with the TR-20 model. The calibration data included times of concentration from 0.25 to 2.0 hours, and testing indicates that the method is reasonably accurate for times of concentration as low as 0.10 hours. Once the al- lowable peak discharge rate from a SWM basin has been set, variations in the SWM basin design parameters do not significantly affect the SWM basin outflow hydrograph; therefore, the planning method does not require that a SWM basin be designed. The planning method is a fast and reasonably accurate way to determine whether or not installation of a SWM basin will increase flooding at downstream points.

81 3

ACKNOWLEDGMENTS

The research on which thia report is based was supported, in part, by the Tidewater Administration, Department of Natural Resources, Annapolis, Maryland.

LITERATURE CITED

Bondelid, T. R. and R. H. McCuen, 1980. A Computerized Method for Designing Stormwater Management Basins. Tech. Report, Depart- ment of Civil Engineering, University of Maryland.

McCuen, R. H., 1974. A Regional Approach to Urban Storm Water De- tention. Geophysical Research Letters 1(7):321-322.

McCuen, R. H., 1979. Downstream Effectsof Stormwater Management. Journal of Water Resources Planning and Management, ASCE 105

Soil Conservation Service, 1972. National Engineering Handbook, Sec- tion 4, Hydrology. U.S. Dept. of Agriculture, Washington, D.C.

(HY 3): 1 34 3-1 35 6.


a planning method for evaluating downstream effects of detention basins

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